Nuclear power reactors rely on neutron absorbing rods to control reactivity. Traditionally, rods are inserted into the reactor core in sets, with a set of rods for each fuel assembly with control rods. The control rods may be moved within the core to control (at least partially) the nuclear fission that occurs in the core. Some reactor designs have a control rod drive mechanism located on one side of a primary system pressure boundary, while the control rods are located on an opposite side of the pressure boundary within the core. Other designs have a drive mechanism be part of the pressure boundary. During normal operation, the rods are positioned within the core by the drive mechanism providing either a force to insert the rods further into the core or a force to remove the control rods from the core. The drive mechanism also may hold the control rods at several different degrees of insertion within the core.